1. Field of the Invention
The present invention relates to a piezoelectric thin film used in wide fields and a device to which the piezoelectric thin film is applied, such as a thin film resonator, a thin film VCO (Voltage Control Oscillator), a thin film filter, a duplexer, or various sensors for use in a mobile communication device or the like.
2. Description of the Related Art
Devices using piezoelectric phenomena are used in a wide range of fields. While downsizing and power saving of mobile devices progress, use of SAW (Surface Acoustic Wave) devices as RF and IF filters has been expanding. SAW filters have improved in designs and production techniques to meet users' demands for strict specifications. However, improvements in characteristics of SAW filters are approaching limits as used frequencies rise higher. This necessitates a great technical innovation in both aspects of miniaturization in formation of electrodes and maintenance of stable output. On the other side, in an FBAR (Thin Film Bulk Acoustic Resonator) and SBAR (Stacked Thin Film Bulk Acoustic Wave Resonators and Filters) using thickness vibration of a piezoelectric thin film, a thin film mainly made of piezoelectric material and electrodes for driving the thin film are formed on a thin support film provided on a substrate. These resonators and filters can generate basic resonance at a gigahertz band. If a filter is constituted by an FBAR or SBAR, remarkable downsizing can be achieved, and low loss and wide-band operation are possible. Besides, the FBAR or SBAR can be integrated with a semiconductor integrated circuit. Therefore, future application of FBAR and SBAR to very small devices for mobile phone is expected.
A piezoelectric thin film device such as FBAR and SBAR utilizing an acoustic wave is manufactured as follows. A base film made of a dielectric thin film, conductive thin film or a laminated film thereof is formed on a substrate made of single crystal semiconductor such as silicon or poly-crystalline diamond formed on a silicon wafer, by various thin film forming methods. A piezoelectric thin film is formed on the base film, and further, an upper structure is formed upon necessity. After forming each film or all films, a physical or chemical treatment is carried out on each film, thereby to perform microfabrication and patterning. A bridge structure in which a part positioning below a vibration portion is removed from the substrate is prepared by anisotropic etching. Finally, each one device unit is thereafter separated to obtain a piezoelectric thin film device.
For example, a piezoelectric thin film device disclosed in JP-A-60-142607 is manufactured by forming a base film, a lower electrode, a piezoelectric thin film, and an upper electrode on a substrate, and thereafter by removing a part of the substrate existing below a part of a vibration portion from the back side of the substrate, to form a via hole. Materials for the piezoelectric thin film of the piezoelectric thin film device may include aluminum nitride (AlN), zinc oxide (ZnO), cadmium sulfide (CdS), lead titanate (PT(PbTiO3)), lead zirconate titanate (PZT (Pb(Zr,Ti)O3)). In particular, AlN with high propagation speed of the acoustic wave is suitable as a piezoelectric material for a thin film resonator and thin film filter which operate at a high frequency band.
JP-A-2000-244030 discloses a piezoelectric thin film device characterized by a conductive thin film containing titanium and at least one of platinum, iridium, ruthenium and ruthenium oxide as a major component. However, performance of the piezoelectric thin film device is not described herein at all.
JP-A-2005-073175 discloses a piezoelectric thin film resonator in which a layer of ruthenium or ruthenium alloy is included in an upper or lower electrode. However, requirement for the characteristics of the upper and lower electrodes which are necessary for fabricating a high performance piezoelectric thin film resonator are not clarified herein.